Electrolytic silver refining is normally carried out according to the Moebius or Balbach-Thum processes. The common contaminating metals, in particular copper, are an impediment to the refining process. These contaminating metals are dissolved and concentrated in the electrolyte. In typical silver nitrate baths used for electrolytic silver refining, a copper content of about 60 g/L is considered to be the maximum permissible value. In excess of this value, cathodic deposition of copper may occur whereby Cu salts are trapped in the refined silver so that the purity requirements are no longer met. In situations where there is an excess of copper, entrapment of metal salts also occurs. Therefore, the used electrolyte must be restored to being “fresh” electrolyte having a lower Cu content from time to time.
In the past, silver refining has been performed by electrolyte recycling. The electrolyte recycling, i.e., removal of contaminating metals was typically accomplished by solvent extraction techniques. Swiss Patent No 614,238 describes a method employing solvent extraction for the removal of copper from silver nitrate electrolytes. The extractants used in this patent are commonly chelating agents such as phenoximes or quinolines, and remove substantially all of the copper, and associated metals such as nickel, from the silver nitrate electrolyte.
In silver refining electrolysis, it is necessary that the electrolyte have silver ions in solution. Due to the simultaneous dissolution of Cu and Ag at the anode of the refining cell, more Ag is deposited at the cathode and removed from the electrolyte than is dissolved at the anode. As a result, the electrolyte becomes deficient in silver and more concentrated in copper. To compensate for the depletion of Ag in the electrolyte, pure silver nitrate (in solution form) must be added to the regenerated electrolyte. This is a handicap for the process, as pure silver nitrate is an expensive product.
It is well known that copper and copper-containing compounds are a common contaminant in the silver nitrate/nitric acid solution that is used as an electrolyte in a Moebius-type silver refining process. It would be desirable to provide a method for effectively and relatively inexpensively removing this very soluble copper from a nitric acid and silver nitrate based-solution.
U.S. Pat. No. 3,851,047 describes a hydrometallurgical process whereby a nitric acid solution which contains ferric nitrate is treated by the process consisting of: 1) flash evaporation of the solution at about the boiling temperature of the azeotropic mixture to produce an “over head” of substantially pure nitric acid which is collected and a “bottom” of ferric nitrate, other nitrates, and ferric hydroxide, and 2) hydrolyzing the “bottom” to convert the ferric nitrates to nitric acid. Such nitric acids may be further collected and concentrated for re-use of the nitric acid.
U.S. Pat. No. 5,441,712 describes a hydrometallurgical process for producing zinc oxide which includes the step of producing a solution of zinc nitrate (which is soluble). Following which, the zinc nitrate solution is subjected to thermal hydrolysis to decompose very soluble zinc nitrate to substantially insoluble zinc oxide, the zinc oxide being thereafter recovered by an appropriate process such as by filtration. This reference further describes a process whereby water vapor formed during the process recombines with the gaseous nitric oxides produced during the pyrohydrolysis step to form nitric acid of acceptable strength for re-use in the process of the invention.
It would be desirable to provide a system to regenerate the nitric acid and silver nitrate based electrolyte that is required for the electrolysis of silver according to a Moebius-type process, especially if such a system was provided with an apparatus and reactants that are easily available and inexpensive in comparison to those used and described in the prior art.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.